The prior art shows numerous mechanisms wherein springs are used to provide a counter-weighting force, to ease the labor of lifting a weight against the force of gravity. See U.S. Pat. No. 3,820,478 to Bergenthal directed to an article support stand for lifting a stack of trays, dishes or the like; U.S. Pat. No. 2,168,209 to Haupt showing a spring counter-weight for lifting the weight of an x-ray apparatus or the like; U.S. Pat. No. 4,760,622 to Rohrman showing a spring counter-weighting system for a window sash or the like; and U.S. Pat. Nos. 4,685,648 to Dobner et al and 4,684,225 to Clark et al relating to spring counter-weight mechanisms and related focusing mechanisms for optical microscopes. Also of interest are Rouverol U.S. Pat. No. 2,924,411 showing a spring support for piping, Leunig U.S. Pat. No. 4,389,228 showing a spring-biased constant tensioning device for maintaining a filter bag assembly in a bag house, and U.S. Pat. No. 4,351,245 to Laporte showing a spring counter-weighting system for a vertically adjustable table top.
As is well known, the force exerted by the typical tension spring varies linearly with its extension. Thus, if the first inch of extension of the spring requires a force of x, the second inch of extension requires 2x, the third inch of extension requires 3x and so on. If it is desired to employ the spring to provide a constant counter-weighting force, as is normally appropriate, some mechanism must be found to maintain the effective counter-weighting force constant despite the linearly increasing force exerted by the spring.
To this end, the art shows numerous mechanisms whereby a "snail cam" or similar device for continually decreasing the mechanical advantage of a lever is employed to convert the linearly increasing force exerted by a tension spring as it is extended to a constant force for counter-weighting moving assemblies. See, for example, the Haupt patent discussed above at FIG. 4, the Dobner patent at FIGS. 1 and 5, and the Laporte patent at FIG. 2.
The snail cam devices shown in the prior art for converting the linearly-increasing force exerted by a tension spring to be constant over a substantial range of extension of the spring typically require means for adjusting the preload on the spring. The prior art fails to teach satisfactory means for adjusting the spring preload. More particularly, in most circumstances when a device using a spring for counter-weighting purposes is assembled, the spring must be given an initial tension or preload to provide an initial force substantially equivalent to the load to be counter-weighted. Commonly a threaded member is provided to adjust the spring tension. As the threaded member is turned with respect to a tapped bore fixed to the structure of the device, the spring is gradually extended until the total force exerted by the spring is substantially equal to the load to be counter-weighted. See for example Bergenthal at 114; Haupt at column 2, lines 16-21; Rohrman at element 32, FIG. 5; Dobner at column 4, lines 35-38; and Laporte at column 3, lines 55-59.
Rouverol shows in FIGS. 14 and 15 the use of a spiral spring for exerting a variable force through the intermediation of a snail cam surface 238. Preload in this system is provided by loosening a bolt 250 securing the fixed end of the spring, moving the end of the spiral spring, and retightening the bolt; see column 11, lines 15-54.
It will thus be appreciated that the methods provided in the prior art for adjusting the preload or static force of springs used to provide counter-weighting force all require tools and are relatively complex. It would be preferable to provide a spring counter-weighting device including provision for simple compensation of variation in the load, not requiring tools for adjustment.
In addition to the devices shown in the patents discussed above, there are commercially available "tool balancers" comprising a housing supporting a sheave on which is wound a cable. The sheave is spring-loaded to exert a constant tension on the cable, approximating the weight of a tool supported by the cable, so that its user need not heft the weight of the tool throughout the workday. A wide range of such devices are offered, each intended for use with tools of particular weight. It would be desirable to provide a tool balancer adjustable to support tools of widely varying weights, not requiring tools for adjustment, so that a single tool balancer would be suitable for use with a range of tools.
One application of the load compensator of the invention is in connection with tables or desks having spring counter-weighting mechanisms for lifting the desk top vertically. More specifically, the present invention is directed to a load compensator for adjusting the counter-weighting force provided by a tension spring, for example, as weight is added to the table. Such tables are particularly desired for supporting computer equipment. Thus, for example, the table may be supplied with a spring preloaded to effectively counter-weight the basic mass of the table top and supporting structure, so that the table top can conveniently be raised and lowered as desired. The user may want to add a basic computer system and will need to adjust the spring force to insure that the height of the table top continues to be readily adjustable. He may then desire to add a printer or another heavy device. He may then find himself in the habit of keeping heavy books, lamps, etc. on his table top. The counter-weighting force must be adjusted in each case in order that the table top may be readily raised or lowered. The prior art discussed above does not provide a suitable manually operable mechanism for thus compensating for variation of the load in a spring counter-weighted mechanism, particularly one incorporating a snail cam for providing a constant counter-weighting force.